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The Miller Center of Public Affairs at the University of Virginia has a repository of presidential tapes and transcripts featuring Nixon and Haldeman discussing Mark Felt.

The Presidential Recordings Program (http://millercenter.org/academic/presidentialrecordings) at the University of Virginia’s Miller Center of Public Affairs (http://www.millercenter.org) has a spectacular Watergate collection of Nixon tapes and transcripts online:

• http://tapes.millercenter.virginia.edu/transcript/nixon/watergate

In particular, there’s this Oct. 19, 1972 conversation between Nixon and Robert Haldeman, in which Haldeman reports that he’s heard from a confidential source that Mark Felt is leaking information to the news media about the FBI’s investigation into the Watergate break-in:
• http://tapes.millercenter.virginia.edu/clips/1972_1019_felt/

Nixon also mentions Felt in the June 23, 1972 “Smoking Gun” conversation with Haldeman:
• http://tapes.millercenter.virginia.edu/transcript/nixon/smoking-gun

PRP scholar Ken Hughes puts the Nixon tapes in perspective in an online essay, Why Didn’t Nixon Burn the Tapes? — which also includes tapes and transcripts of Nixon-Haldeman conversations.